White balance refers to the capture of the color temperature reflected from a photographic subject and the adjustment of camera settings based on the captured color temperature. The camera settings are adjusted such that the colors in the final print substantially match those in reality.
Most digital cameras have an automatic white balance function. However, automatic white balance readings may sometimes fail to correctly distinguish various types, amount and directions of light falling on a scene.
One solution to the problem is to base the white balance setting on a custom setting, typically a scene exposed at 18% gray. This can be done by capturing a reference photo, which typically includes a neutral scene (e.g., a card printed at 18% gray) or a neutral-toned scene (e.g., sidewalk). The reference photo is saved in the camera and used to set the custom white balance in the camera. The reference photo may be taken using a white balance filter lens. However, conventional white balance filter lenses are typically designed to allow easy placement and removal, and are not designed to securely lock to the camera as camera lens protection devices. As a result, conventional white balance filters typically are insecurely placed onto the camera lens and often have the problem of falling off easily. For example, some conventional white balance filter lenses (e.g., ExpoDisc, a product of ExpoImaging, Inc. of Morgan Hill, Calif.) uses ball bearings to attach to the inner rim of the lens housing. The ball bearing design allows the ExpoDisk to be easily put on and taken off. However, a standard inner rim of the lens housing for lens filter attachment is a screw (or thread) type machining. Ball bearing locks cannot accurately line up consistently with the threads as the threads are slanted by nature.
Therefore, a conventional white balance filter lens is generally not an effective lens protection device. Any contact with the inside of a camera bag will likely separate the filter lens from the front of the camera lens, and may even cause damage to the front element of the camera lens, the camera lens body, or the camera itself. Thus, it is generally necessary for a photographer to carry a white balance filter lens as an additional device in the camera bag, and to perform the steps of removing the camera cap and placing the white balance filter lens before taking a reference photo.